Unfortunately you have fallen into the trap of FL sizing your brass way too much from the beginning. High pressures are NOT the cause of this case stretching, it is ONLY caused by excessive sizing of the shoulder. SAAMI specs dictated for factory loaded ammo is fine and dandy, but once it's been fired in your chamber it's a completely different kettle of fish. As you've found out, (the hard way!), you must be very careful with how much you bump the shoulder back, .002" is maximum if you want your brass to last more than a few firings.

Your brass can be salvaged, run it through your body die with the press camming over, there should be no gap between the shellholder and die bottom when doing this. This will reduce the dimensions just above the belt, no matter what anyone tells you!
Once this is accomplished, you will need to fireform the brass back to your chamber dimensions with a mild load, a true fireforming load without a projectile would be better, but this will suffice.

Once you have fireformed the brass, measure the base to shoulder length and adjust your body die to bump the shoulder .002", this will allow the case to headspace off the shoulder.
If this is done correctly, you will eliminate that bulge forever, it only occurs when the brass stretches just above the belt due to excessive shoulder sizing. Under no circumstances do you want a belted bottleneck cartridge to headspace off the belt, other than the 300H+H, 375H+H, 458WM or 458Lott.

BTW, even though there are advocates for the Larry Willis die, I do not believe in it, it's a complete unnecessary gimmick if you size your cases correctly to headspace off the shoulder. I am yet to run across this 'bulge' in any of my belted cartridges and, I load for a hell of a lot of 'em.

I dont think it is a gimmick...

That said I agree with you if you set up your fl sizer correctly or better yet beck size and run the fl sizer only when necessarily you should be fine.

__________________

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You need a mild load of a medium burning rate powder for a fireforming load, RE25 is way too slow for this. Something in the range of H4198 with a 125gr/130gr projectile and a starting load.

The reason I recommend a fireforming load is that it will stretch the shoulder back to chamber specs without overly stressing the area above the web of the case, this is where any stretching will take place due to the excessive shoulder headspace.
In this instance, you want mild pressures to stretch the shoulder, or you will get incipient case head separations within a couple of firings.

I know this is technical, but a fire forming load without a projectile is what I recommend.
If you can get your hands on some fast pistol powder, such as Clays or red dot, and use just enough to blow the shoulders forward, you'll have to experiment with the charge weight, then you should be good to go. To form a case, I use Clays.

Now my 375Weatherby takes 30gr's to give full rounded shoulders, this is probably too much for a 300WM case. I would try 15-20gr as a starting point, and increase in 5gr increments until the case shoulder is sharply defined and measures .0075" longer. There is no risk of an overload, no projectile is engaging the rifling!

This is the part you will probably get a giggle out of!

Once you have loaded your cases, push an appropriately sized piece of pumpkin or potato, for neck length, into the case mouth and fire at the ground or downrange. Sometimes a little molten candle wax dripped over the pumpkin/potato helps to get slightly higher pressures, but is not totally necessary.

Like Magnum said, it is how you size not he die. I have 10 reloads on 340 WM, about half are fairly hot compressed loads that I DONT SHOOT in summer. But great for elk season. I use a rcbs die and just bump shoulder .

ok so I have one more question. ok no promises there will only be one...

If fire forming these cases was going to solve this problem, why are my cases that have been fired(the other 20%) not showing an improvement in the bulge area? Not being a smart A, just trying to figure out how a light charge is going to affect/effect the brass differently than a "normal" load...

and here are the current powders I have. If one of these will work I can get started tonight if not I will have to make a trip in the AM....

R22
IMR 4350
benchmark
H4831
varget
bl-c(2)

__________________
"Killen is my business and business is GOOD!!!"
Major Benson Winerford Payne

Use about 14 grs. of red dot, fill the rest with cream of wheat, and put a cotton plug in the neck. This will blow the shoulder back where it needs to be without stretching the web as others have said. Then you can size the shoulder back about a thou.....Rich

Well I am not here to argue but I disagree with the statement that one will not get a magnum bulge if one headspaces off the shoulder .001-.002. I have found that some rifles have this problem on rare occasion, such as one 300 Win Mag custom rifle I have reloaded for that never showed the issue in 4-5 case firings.

2 other Magnums show it every 2 firings. One 300 Win mag will show the issue after 3-4. A 338 win mag shows signs after 2 and a 7mm Rem Mag shows almost 30% of the cases after each firing.

Its no myth and is well known.
Anyway, everyone has their opinion and I have no problem with that either.
P.S. I always headspace every single case .001-.002. Every case.

Also the Post's author is speaking of magnum bulge and not insipient head separation. The two are not denotatively the same.

I'm going to throw this out there for consideration. I ran across this article that deals with sizing belted cases and how to avoid excessive case stretch. May not help with the bulge you are seeing but will go a long ways in increasing case life and preventing case head separation.Belted Case Blues.